avformat/mxfenc: Never set codec_ul UID to NULL

mxf distinguishes codec profiles by different UIDs and therefore needs
to check that the input is actually compatible with mxf (i.e. if there
is a defined UID for it). If not, then sometimes the UID would be set to
NULL and writing the (video) packet would fail. Yet the following audio
packet would trigger writing the header (which has been postponed because
the UID is not known at the start) and if the UID is NULL, this can lead
to segfaults. This commit therefore stops setting the UID to NULL if the
input is incompatible with mxf (it has initially been set to a generic
value in mxf_write_header()).

Fixes #7993.

Reviewed-by: Tomas Härdin <tjoppen@acc.umu.se>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavformat/mxfenc.c

@@ -2020,7 +2020,6 @@ static int mxf_parse_prores_frame(AVFormatContext *s, AVStream *st, AVPacket *pk
     if (mxf->header_written)
         return 1;
 
-    sc->codec_ul = NULL;
     profile = st->codecpar->profile;
     for (i = 0; i < FF_ARRAY_ELEMS(mxf_prores_codec_uls); i++) {
         if (profile == mxf_prores_codec_uls[i].profile) {
@@ -2028,7 +2027,7 @@ static int mxf_parse_prores_frame(AVFormatContext *s, AVStream *st, AVPacket *pk
             break;
         }
     }
-    if (!sc->codec_ul)
+    if (i == FF_ARRAY_ELEMS(mxf_prores_codec_uls))
         return 0;
 
     sc->frame_size = pkt->size;
@@ -2074,7 +2073,6 @@ static int mxf_parse_dnxhd_frame(AVFormatContext *s, AVStream *st, AVPacket *pkt
     if (pkt->size < 43)
         return 0;
 
-    sc->codec_ul = NULL;
     cid = AV_RB32(pkt->data + 0x28);
     for (i = 0; i < FF_ARRAY_ELEMS(mxf_dnxhd_codec_uls); i++) {
         if (cid == mxf_dnxhd_codec_uls[i].cid) {
@@ -2082,7 +2080,7 @@ static int mxf_parse_dnxhd_frame(AVFormatContext *s, AVStream *st, AVPacket *pkt
             break;
         }
     }
-    if (!sc->codec_ul)
+    if (i == FF_ARRAY_ELEMS(mxf_dnxhd_codec_uls))
         return 0;
 
     sc->component_depth = 0;
@@ -2245,6 +2243,7 @@ static int mxf_parse_h264_frame(AVFormatContext *s, AVStream *st,
     const uint8_t *buf = pkt->data;
     const uint8_t *buf_end = pkt->data + pkt->size;
     const uint8_t *nal_end;
+    const UID *codec_ul = NULL;
     uint32_t state = -1;
     int extra_size = 512; // support AVC Intra files without SPS/PPS header
     int i, frame_size, slice_type, has_sps = 0, intra_only = 0, ret;
@@ -2315,12 +2314,11 @@ static int mxf_parse_h264_frame(AVFormatContext *s, AVStream *st,
 
     if (!has_sps)
         sc->interlaced = st->codecpar->field_order != AV_FIELD_PROGRESSIVE ? 1 : 0;
-    sc->codec_ul = NULL;
     frame_size = pkt->size + extra_size;
 
     for (i = 0; i < FF_ARRAY_ELEMS(mxf_h264_codec_uls); i++) {
         if (frame_size == mxf_h264_codec_uls[i].frame_size && sc->interlaced == mxf_h264_codec_uls[i].interlaced) {
-            sc->codec_ul = &mxf_h264_codec_uls[i].uid;
+            codec_ul = &mxf_h264_codec_uls[i].uid;
             sc->component_depth = 10; // AVC Intra is always 10 Bit
             sc->aspect_ratio = (AVRational){ 16, 9 }; // 16:9 is mandatory for broadcast HD
             st->codecpar->profile = mxf_h264_codec_uls[i].profile;
@@ -2334,17 +2332,18 @@ static int mxf_parse_h264_frame(AVFormatContext *s, AVStream *st,
                    mxf_h264_codec_uls[i].profile == sps->profile_idc &&
                    (mxf_h264_codec_uls[i].intra_only < 0 ||
                     mxf_h264_codec_uls[i].intra_only == intra_only)) {
-            sc->codec_ul = &mxf_h264_codec_uls[i].uid;
+            codec_ul = &mxf_h264_codec_uls[i].uid;
             st->codecpar->profile = sps->profile_idc;
             st->codecpar->level = sps->level_idc;
             // continue to check for avc intra
         }
     }
 
-    if (!sc->codec_ul) {
+    if (!codec_ul) {
         av_log(s, AV_LOG_ERROR, "h264 profile not supported\n");
         return 0;
     }
+    sc->codec_ul = codec_ul;
 
     return 1;
 }
@@ -2441,9 +2440,13 @@ static int mxf_parse_mpeg2_frame(AVFormatContext *s, AVStream *st,
             }
         }
     }
-    if (s->oformat != &ff_mxf_d10_muxer)
-        sc->codec_ul = mxf_get_mpeg2_codec_ul(st->codecpar);
-    return !!sc->codec_ul;
+    if (s->oformat != &ff_mxf_d10_muxer) {
+        const UID *codec_ul = mxf_get_mpeg2_codec_ul(st->codecpar);
+        if (!codec_ul)
+            return 0;
+        sc->codec_ul = codec_ul;
+    }
+    return 1;
 }
 
 static uint64_t mxf_parse_timestamp(int64_t timestamp64)